The imperfections of the methods of fabricating parts lead to dispersion in terms of the shapes and dimensions of the geometrical envelope of the parts produced and therefore discrepancies between the geometry of their nominal surfaces, i.e. the geometry of their intended theoretical surfaces, and the geometry of their real surfaces obtained after fabrication.
Complex mechanical structures are often produced by assembling a plurality of parts held together by mechanical connections, which assembly operations are generally carried out at one or more assembly stations.
Because of the differences between the geometry of the nominal surfaces and the geometry of the real surfaces of the parts, these assembly stations necessitate means for modification and geometrical adjustment of the shapes of the parts that have to be assembled, such as inserting shims, filling with mastic or reworking.
The production of a mechanical connection termed “complete”, i.e. with no degree of freedom, between two parts imposes the surfaces of the two parts that are face-to-face in the assembly bearing one on the other in as perfect a manner as possible, i.e. so as to minimize the residual clearance at the interface of these surfaces and the stresses introduced by deformations of the parts during assembly. This assumes that the two surfaces are geometrically complementary at their interface. By “interface” is meant the area of junction of the parts, corresponding to the respective facing surfaces of those parts to be assembled.
According to a first known method, the discrepancy at the interface is eliminated by the local deformation of one or both parts caused by the forces introduced during the production of the connection. The deformation, which may more particularly affect the less rigid part, is accompanied by modifications of the mechanical stress state of the parts.
This modification of the stress state must be limited so as not to affect, in particular, the mechanical strength of the parts and therefore of the mechanical structure. Moreover, in the case of rigid parts, it is not always possible to guarantee the deformation of the part to ensure the intimate contact of the assembled parts.
According to another method, a polymerizable filler mastic is deposited at the interface of the two parts and when the parts are pre-assembled flows into the areas in which contact between the parts is not achieved and thus fills the voids that would be formed in the absence of mastic.
However, the use of mastics of this kind is irksome and is not always possible, depending on the forces that have to be transmitted in the joint and the assembly elements used. Moreover, mastics having appropriate structural properties are dense and this can result in an increase in the weight of the assembled structure, which is all the more problematic in that it is not controlled.
It is therefore desirable to control the geometry of the surfaces at the interface of the parts to ensure an acceptable level of deformation and therefore of modification of the stress state.
This acceptable level of deformation is defined when designing the structure by assembly specifications and may be expressed by specified limits on the permissible maximum geometrical discrepancies of the surfaces at the interface of the parts to be assembled.
In order to limit these discrepancies at the interface of the surfaces of two parts to be assembled, it is known to measure the differences between the real and nominal surface geometries of a first part in order to determine the geometry of a nominal surface of a second part to be fabricated or in order to correct the geometry of a nominal surface of a second part that has already been fabricated. This is so that the differences between the geometries of the nominal and real surfaces of the first part are compensated by the geometry of the nominal surface of the second part. The geometry of the nominal surface of the second part is therefore determined so that it is complementary to the geometry of the real surface of the first part.
The differences between the geometry of the real and nominal surfaces are generally determined by measuring the surface of the first part, for example using a three-dimensional measuring machine. Moreover, the fabrication of the second part or the correction of the geometry of its nominal surface is typically effected by removing material, i.e. by machining.
The operations of fabrication of a part or of correction of the geometry of its nominal surface are relatively complex to implement if that part is a thin part, for example a metal sheet, in that the removal of material can significantly reduce the mechanical strength of the part. Moreover, the correction operations may be impossible to carry out without affecting the integrity of the part if the thickness of the part becomes less than a minimum necessary for the transmission of the designed forces in the part.
Moreover, these operations necessitate supplementary manipulations of the parts to be assembled and therefore a significant loss of time on the production or assembly line.